There will eventually be an underground with various layers. :) I need to get multiplayer working first but it's one of the things that is definitely planned.
I'm thinking that once multiplayer is in, I might make the solo version free? Not sure yet, but that might be a good way to get people making a lot of mods once the game is in a pretty polished state.
Thanks for the compliments! If you like, check out the Discord at discord.gg/modlands, or the Twitch channel; there's a lot of new procedurally generated stuff going into the game now, so it's fun streams. :)